Reply to comment
A simple solution
The simplest server room thermometer is the basic hand held thermometer most people will be familiar with. A handheld thermometer is very useful for discovering the ambient temperature in your server room and for finding any hot spots that may be lerking around. The main problem with a handheld thermometer is that they have no memory, or at best very limited memory.
A bit closer...
The next best solution for monitoring your server room temperature is a data logger. Data loggers are inexpensive units with either a built in temperature sensor or one or more external temperature probes. The benefit that data loggers bring is that they are able to record temperatures for long periods (typically up to 20-40,000 readings) without manual intervention and without external power. The main problem with data loggers is that their ability to alert you to a heat anomaly is very limited. Usually, a data logger will only indicate an alarm condition on the data logger itself. In order to detect the alarm condition you would need to manually inspect the data logger unit on a peridic basis. Hardly ideal.
There is a better way!
Fortunately, there are a range of devices called environment monitors that are able to measure the temperature in your server room and alert you to heat related issues. Typically environment monitors are able to measure the temperature periodically (precisely how close you want the measurements is up to you), log the readings and graph of the results.
Most environment monitors are also able to alert you when the temperature goes above or below a threshold you specify. So, you can know when the temperature in your server room goes above 25°C and be alerted via email or text message usually with a handy graph of the recent temperatures.
Environment monitors have richer alerting options because they are network enabled, meaning that they connect to your network just like a server or desktop computer.
Most environment monitors also provide a web server, so you can view the temperature in real time from your own desktop or laptop. Many network thermometers also support the popular SNMP protocol, meaning that you can integrate them into your favourite network management system.
Many environment monitors are designed to be integrated into the server room or data centre environments fitting into the same racks as your servers and networking equipment.
For a more comprehensive guide to environment monitoring in the server room, please read the IT Environment Monitor Buyer's Guide.